RESIDENTS in south-eastern Sydney were told it was safe to drink their tap water, hours before Sydney Water had received the test results showing what had caused the widespread contamination of that drinking water in December last year.

Dr Wayne Smith, the NSW Health director of environmental health, said he was confident in Sydney Water's assessment that the contamination was from bitumen coming into contact with the water and it would be safe to drink after water pipes were flushed clean.

But Sydney Water has been accused of covering up the real source of the contamination, which occurred on December 28.

The revelations that residents were given the all clear several hours before it was known what was in the water comes as Fairfax Media has also discovered that Sydney Water had kept secret the results of one water sample which was taken from a local manufacturer's water storage tank.

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Sydney Water had previously released only three results showing low levels of contaminants known as PAHs in the water, which it said backed its claim the contamination was caused by water coming into contact with the bitumen lining of water mains pipes during maintenance work in the suburb of Rosebery.

But it had not revealed the fourth sample from the storage tank in Pagewood that showed the levels of the PAHs were more twice the level of other water samples taken in surrounding streets.

Analytical chemist Ian Brown said the much higher levels of contamination shown in the storage tank sample should have been a red flag indicating that the source of the contamination may have been close to the tank in Pagewood and not at Rosebery as previously thought.

Resident Chantal Snell who has been fighting to find out the truth about the incident said it is concerning Sydney Water did not release the full range of information. ''That creates further suspicion,'' she said.

Sydney Water took water sample tests on the night residents reported the water smelling like gas or petrol, feeling oily and people experiencing vomiting, dizziness and numbness in the mouth for hours after drinking it.

The first results from the samples were not known until almost 24 hours later, at 11pm on December 29. But Sydney Water had already told NSW Health the water was safe.

It has since emerged that Sydney Water only took water samples for testing in the areas of the complaints, which happened to be streets around the Botany Industrial Park, an area known to have ground and water contamination issues.

Sydney Water did not take any water samples from mains pipes in Rosebery or the suburbs in between. A spokeswoman for Sydney Water said last week it is now understood that the majority of the contaminated water had travelled down one of the mains pipes to the suburbs of Pagewood, Botany, Banksmeadow and Port Botany.

The contamination scare sparked calls for a thorough investigation into the incident and prompted an angry response from the Premier, Barry O'Farrell, about the role of Sydney Water. However, Mr O'Farrell said he was confident the water was now safe to drink.

Questions asked of Sydney Water last week revealed it had only done its standard drinking water scan on the contaminated water and it had not looked for any other possible sources of contamination.

Residents in the Pagewood area fear the contamination came from the nearby Botany Industrial Park where contaminated water is being treated and recycled for industrial uses.

Sydney Water has admitted it did not ask its laboratory to test for contamination from trade waste or waste water discharge. Nor did it ask for tests to look for unexpected volatile gases.

But given the characteristics of the contaminated water that residents were reporting, which included oiliness, a petrol smell and the apparent evidence of a flammable gas, a scientist from Macquarie University's department of chemistry, said the test should have included ''headspace'' tests for very flammable materials which are suspected of being the real culprits.